If you choose kg/kmol or g/g_mole preffered units of molar_weight_constant, kg/mol dissappears. The problem is libascend returns one less unit (function library.getUnits()). It returns 450 instead 451 when no preffered units are selected.

Steps To Reproduce

1. Run Ascend with airprops model
2. Select M and click 'Select units'
3. Choose kg/kmol
4. Click apply and close
5. Close Ascend
6. Start Ascend once again
7. Select M and click 'Select units'
There is one less unit.